Final answer:
The density of the universe determines whether it will expand forever or eventually collapse. A low density allows for indefinite expansion, while a higher density can lead to a slowdown in expansion and potential collapse.
Step-by-step explanation:
The density of the universe plays a crucial role in determining its development. If the average density of the universe is low, there is less mass and gravity, allowing for expansion that can continue indefinitely.
On the other hand, a higher average density means more mass and gravity, which can slow down the expansion and eventually lead to the universe collapsing. An extremely high density could even cause the universe to collapse again.